Compounds containing more than one type of functional group are often referred to as multifunctional compounds. The type of functional groups determines the properties and applications of the compounds. When multifunctional compounds contain moisture-curable functional groups such as alkoxysilane groups, for example, the multifunctional compounds are useful in applications where polymer curing is effected by exposure of polymers containing the multifunctional compounds to moisture. Alkoxysilane groups can form siloxane (--Si--O--Si--) linkages in the presence of atmospheric moisture. Siloxane linkages not only form a polymer network, but they also improve adhesion of the polymers to non-porous surfaces, such as glass.
Multifunctional compounds containing moisture-curable functional groups have found use in many applications. For example, U.S. Pat. No. 5,587,502 describes the use of multifunctional compounds in the preparation of moisture-curable adhesives, sealants (e.g., automobile seam sealants), putties, and the like. The multifunctional compounds therein comprise both hydroxy and alkoxysilane moieties. The hydroxy functionality can react with isocyanate-functional materials to form alkoxysilane-functional polyurethanes. See also, PCT Publication No. WO98/18844, wherein multifunctional compounds comprising both hydroxy and alkoxysilane moieties are reacted with an isocyanate-functional poly(ether-urethane) to form an alkoxysilane-functional poly(ether-urethane). The cured compositions therefrom are also particularly useful as sealants.
U.S. Pat. No. 5,717,125 discloses a wide variety of multifunctional compounds that are both hydrolyzable and polymerizable to form both inorganic and organic networks, respectively, throughout the resulting composition. The compositions therein are purportedly particularly useful in dental applications. However, it is not always desirable to include polymerizable groups in the multifunctional compounds.
Multifunctional compounds bearing both amine and alkoxysilane moieties are well known. See, for example, U.S. Pat. Nos. 3,033,815; 3,627,722; 3,632,557; 3,700,716; 3,979,344; 4,067,844; 4,209,455; 4,628,076; 4,718,944; 4,857,623; 5,174,813; and 5,364,955. Both primary amine-functional alkoxysilanes and secondary amine-functional alkoxysilanes are described therein.
Primary amine-functional alkoxysilanes are often extremely reactive with a variety of electrophiles (e.g., isocyanates, oxirane rings, and anhydrides), resulting in strongly hydrogen-bonded products. For example, the reaction of primary amines with isocyanates is extremely fast and produces dihydrourea linkages. Dihydrourea linkages may disadvantageously increase product viscosities, however, which can hinder processability of the product, subsequent mobility of attached functional groups and reactivity of attached functional groups. Additionally, fast reaction rates, often associated with primary amine-functional alkoxysilanes, are undesirable in many applications. For example, uncontrollable reaction rates can lead to excessive generation of heat, fast gel times, and decreased reaction selectively.
In general, secondary amine-functional alkoxysilanes react more slowly with electrophiles than do the corresponding primary amine-functional alkoxysilanes. Furthermore, hydrogen bonding in their adducts is significantly reduced or eliminated. Examples of commercially available secondary amine-functional silanes include: 3-(N-phenyl)aminopropyltrimethoxysilane; 3-(N-methyl)aminopropyltrimethoxysilane; and 3,3'-iminobis(propyltrimethoxysilane). Methods of preparation are described in U.S. Pat. No. 3,632,557. However, costs of such secondary amine-functional alkoxysilanes, often twice those of their primary amine analogs, limit their commercial application.
U.S. Pat. Nos. 3,033,815 and 4,067,844 disclose secondary amine-functional alkoxysilanes that are formed by a Michael-type reaction of primary amine-functional alkoxysilanes with (meth)acrylate or (meth)acrylonitrile Michael-type receptors. Although these methods produce secondary amine-functional alkoxysilanes at relatively low cost, the reaction products are contaminated with varying quantities of primary and tertiary amine-functional alkoxysilanes.
Preparation of N-alkoxysilylalkyl-aspartic acid diesters from certain amino-alkyl alkoxysilanes and maleic or fumaric acid esters is disclosed in U.S. Pat. No. 5,364,955. These N-alkoxysilylalkyl-aspartic acid diesters react with isocyanates, however, to form polymers containing urea and ester groups. The polymers are reportedly unstable, however, with the urea groups cyclizing to hydantoins (See U.S. Pat. No. 5,756,71). This reaction is illustrated below, wherein X, Z, R, R.sub.2, R.sub.3, R.sub.4, n, and m are as defined therein: ##STR1## As can be seen from the reaction diagram, during cyclization, alcohol is expelled as a by-product. Expelled alcohol can be problematic since it may undesirably slow subsequent moisture-cure of the alkoxysilane moieties. Furthermore, hydantoin formation may lead to undesirable shrinkage of the resulting polymer. See also European Patent Application No. 0 831 108 A1, which discloses N-alkoxysilylalkyl-aspartic acid diesters and polyurethane products therefrom, which polyurethane products are reportedly useful as sealants.
While many multifunctional compounds, particularly secondary amine-functional silanes are known, a further variety of compounds would be desirable to enable tailorability for certain applications. For example, secondary amine-functional silanes that are stable without cyclizing to form hydantoins are desirable for use in applications where shrinkage, for example, from hydantoin formation is undesirable. One such application is the field of sealants, like automobile seam sealers. Furthermore, cost-effectiveness is also generally a consideration when selecting suitable multifunctional compounds for formulating compositions.